This invention relates to integrated circuits, and more particularly to clock buffers and drivers with reduced EMI.
Electronic devices and equipment can often generate interference that creates disturbances in other electronic equipment. For example, a portable computer may emit radiation that interferes with a television receiver. Intentional receivers, such as televisions, cellular phones, pagers, and wireless devices, are often affected by unintentional transmitters that emit electromagnetic radiation. As both types of electronic equipment become more common, interference becomes more noticeable to the consumer.
Electromagnetic interference (EMI) is a measure of the amount of interference that an electronic device (the unintentional transmitter) disturbs an intentional receiver. Government agencies such as the Federal Communication Commission (FCC) strictly regulate the amount of radiation or EMI that an electronic device can generate. Portable devices such as portable personal computers (PC""s) are particularly problematic since these portable devices are easily placed near televisions or other receivers. Cramped office and living spaces often force electronic equipment closer together. Since radiation rapidly increases with smaller distances, smaller and more portable equipment often appear to generate more interference.
Improving technology also worsens the EMI problem. Faster clock rates of higher-speed PC""s generate more radiation. Higher communication rates require that more data bits be transferred in a time period; a higher clock rate and more interference results.
A technique to reduce peak EMI is to vary or modulate the frequency of clocks in the electronic device. This technique known as spread spectrum, since the frequency spectrum of the clock is spread out over a wider range of frequencies. Phase-locked loops (PLL""s) have been developed that vary the control voltage to a voltage-controlled oscillator (VCO), or vary a feedback divisor to modulate the frequency generated. Unfortunately, noise and tracking can be a problem, especially when a clock generator and multiple clock buffer drivers are cascaded.
Another approach to reducing EMI is to insert controllable delays in parallel signal paths. See for example U.S. Pat. No. 6,144,242 by jeong et al., and U.S. Pat. No. 5,675,832 by Ikami et al. This technique is useful for staggering delays among signal lines in a wide data bus so that all signals do not have coincident edges.